This invention relates to a thick film fuse assembly for high reliability applications. These fuses are particularly suitable for use in circuits requiring fuses with a small footprint which are surface mountable. Further, these fuses can be provided in surface mount chip or flip chip packages and are capable of operating at voltage levels above 32 volts D.C. and at amperage ratings greater than 2.0 amps (currently the upper voltage and current limit for commercially available thin film chip fuses). The unique construction of this fuse enables it to provide much higher interrupt rating capacity than commercially available chip fuses of similar size. Additionally, these fuses are suitable for use in environments which may subject the fuse to relatively high levels of mechanical shock and vibration and a broad range of operating temperature. Because the fuse package does not outgas while in a high vacuum, this fuse is ideally suited for space and satellite applications.
Thick film high reliability fuses have, in the past, been constructed on glass or alumina (coated with dielectric barrier) substrates. Fuses manufactured on glass substrates have the following limitations:
a) only low temperature fired thick film materials may be utilized (thick film pastes which fire at temperatures over 500.degree. C. will damage substrate). PA1 b) low temperature thick film inks which must be utilized are less stable at higher fuse operating temperatures (above +125.degree. C.). PA1 c) glass substrate is susceptible to fractures during fuse processing and during normal operation when the fuse is subjected to temperature cycling. PA1 a) the fusible element is generally a thin film material which limits the amount of metallization which can be applied to the fuse element (maximum current rating is only a few amperes). PA1 b) an adequate means of arc suppression is not provided. PA1 c) substrate materials utilized (typically glass) are subject to fractures which may result in the premature failure of the fuse.
Thick film fuses manufactured on alumina (90-99%) substrates are often costly due to the additional processing steps required to make the alumina substrate thermally insulative. This process is accomplished by coating the alumina substrate with a dielectric glass. The manufacturing process of applying the dielectric barrier to the alumina substrate involves the screen printing of a thick film dielectric glass over the surface of the alumina substrate. Several print, dry, and fire cycles are required to build up a sufficient thickness of dielectric glass to provide a thermal insulator for the thick film fuse components which are printed over the dielectric in later manufacturing steps. In the preferred embodiment of a fuse in accordance with the present invention the substrate material utilized is a thermally insulative ceramic, thus eliminating the need to add a thermal barrier to the substrate.
The primary reasons for the voltage and amperage limitations of traditional chip style fuses are as follows:
In the preferred embodiment of a fuse in accordance with the present invention the fuse assembly consists of an insulative substrate in which a low mass thick film element is directly disposed on the substrate. Thick film contact pads electrically connect to the fuse element to permit the attachment of end-caps. A layer of low mass thick film arc suppressive glass covers the fuse element. This construction allows for a higher amperage and voltage rating due to the ability of the arc suppressive glass to prevent restrikes and arcing during an overload current condition. Additionally, the thick film process provides the means for depositing a metallized fuse layer of sufficient thickness to allow for current ratings exceeding 10 amps. Further, copper alloy end-caps allow for greater amperage capacity while isolating the fuse assembly from potential mechanical stresses once the fuse is soldered to a printed circuit board.